1. Field of Invention
This invention is relates to digital to analog converters (DAC's) and in particular resistor voltage divider DAC's.
2. Description of Related Art
Conventional resistor DAC's require 2.sup.n resistors to implement an "n" bit resolution. Thus an eight bit DAC would require 256 resistors, a ten bit DAC 1024 resistors and a twelve bit DAC 4096 resistors. It is easy to see that there is a need to reduce the number of resistors, especially in higher resolution DAC's. In U.S. Pat. No. 4,918,448 (Hauviller et al.) a multistage DAC is described with an extension to provide higher resolution. This design requires 2.sup.n resistors plus 2.sup.p-2 resistors for the extension and provides monotonicity of conversion unimpaired by the extension to "p" additional bits. In U.S. Pat. No. 5,252,975 (Yuasa et al.) is described a resistor DAC having multiple networks and the objective to not have buffer amplifiers interconnecting the various networks. In U.S. Pat. No. 5,495,245 (Ashe) a potentiometer like DAC is described in which there are three resistor divider networks used, two for the most significant bits (MSB) and one for the least significant bits (LSB). The total number of resistors required is 2.sup.n/2 -1 for each MSB resistor divider network and 2.sup.n/2 resistors for the LSB resistor divider network. The resistor values in the LSB network are R/2.sup.n/2 where "R" is the value of the resistors in the MSB networks. In U.S. Pat. No. 5,554,986 (Neidorff) is described a multi-stage resistor DAC designed to produce enhanced accuracy at an increased speed. Each stage converting a portion of the input digital word and the resistors of each subsequent stage having a higher resistor value to reduce loading on the previous stage.
Extending the resolution of a resistor DAC can require a substantial increase in the number of resistors to provide the increased resolution. This is particularly true if the increased resolution is obtained in the straight forward way by adding additional resistors. Various means exist for increasing the resolution of a DAC, but these usually require that the resistors that make up the voltage dividers are not of the same value. This can impair the optimal choice of the value for all of the resistors with respect to accuracy and tolerance of the resistor value. It is important to greatly reduce the number of resistors required to convert a digital word with n+p bits to an analog signal and at the same time have multiple sections of resistors with the value of each resistor in each section being the same.